The present invention is directed toward mechanical and/or chemical-mechanical planarization of microelectronic substrates. More specifically, the invention is related to planarizing machines and to control systems for monitoring and controlling the status of a microelectronic substrate during a planarizing cycle.
Mechanical and chemical-mechanical planarizing processes (collectively xe2x80x9cCMPxe2x80x9d) remove material from the surface of semiconductor wafers, field emission displays or other microelectronic substrates in the production of microelectronic devices and other products. FIG. 1 schematically illustrates a rotary CMP machine 10 with a platen 20, a carrier assembly 30, and a planarizing pad 40. The CMP machine 10 may also have an under-pad 25 attached to an upper surface 22 of the platen 20 and the lower surface of the planarizing pad 40. A drive assembly 26 rotates the platen 20 (indicated by arrow F), or it reciprocates the platen 20 back and forth (indicated by arrow G). Since the planarizing pad 40 is attached to the under-pad 25, the planarizing pad 40 moves with the platen 20 during planarization.
The carrier assembly 30 has a head 32 to which a substrate 12 may be attached, or the substrate 12 may be attached to a resilient pad 34 positioned between the substrate 12 and the head 32. The head 32 may be a free-floating wafer carrier, or the head 32 may be coupled to an actuator assembly 36 that imparts axial and/or rotational motion to the substrate 12 (indicated by arrows H and I, respectively).
The planarizing pad 40 and the planarizing solution 44 define a planarizing medium that mechanically and/or chemically-mechanically removes material from the surface of the substrate 12. The planarizing pad 40 can be a fixed-abrasive planarizing pad in which abrasive particles are fixedly bonded to a suspension material. In fixed-abrasive applications, the planarizing solution is typically a non-abrasive xe2x80x9cclean solutionxe2x80x9d without abrasive particles. In other applications, the planarizing pad 40 can be a non-abrasive pad composed of a polymeric material (e.g., polyurethane), resin, felt or other suitable non-abrasive materials. The planarizing solutions 44 used with the non-abrasive planarizing pads are typically abrasive slurries that have abrasive particles suspended in a liquid.
To planarize the substrate 12 with the CMP machine 10, the carrier assembly 30 presses the substrate 12 face-downward against the polishing medium. More specifically, the carrier assembly 30 generally presses the substrate 12 against the planarizing liquid 44 on the planarizing surface 42 of the planarizing pad 40, and the platen 20 and/or the carrier assembly 30 move to rub the substrate 12 against the planarizing surface 42. As the substrate 12 rubs against the planarizing surface 42, material is removed from the face of the substrate 12.
CMP processes should consistently and accurately produce a uniformly planar surface on the substrate to enable precise fabrication of circuits and photo-patterns. During the construction of transistors, contacts, interconnects and other features, many substrates develop large xe2x80x9cstep heightsxe2x80x9d that create highly topographic surfaces. Such highly topographical surfaces can impair the accuracy of subsequent photolithographic procedures and other processes that are necessary for forming sub-micron features. For example, it is difficult to accurately focus photo patterns to within tolerances approaching 0.1 micron on topographic surfaces because sub-micron photolithographic equipment generally has a very limited depth of field. Thus, CMP processes are often used to transform a topographical surface into a highly uniform, planar surface at various stages of manufacturing microelectronic devices on a substrate.
In the highly competitive semiconductor industry, it is also desirable to maximize the throughput of CMP processing by producing a planar surface on a substrate as quickly as possible. The throughput of CMP processing is a function, at least in part, of the ability to accurately stop CMP processing at a desired endpoint. In a typical CMP process, the desired endpoint is reached when the surface of the substrate is planar and/or when enough material has been removed from the substrate to form discrete components on the substrate (e.g., shallow trench isolation areas, contacts and damascene lines). Accurately stopping CMP processing at a desired endpoint is important for maintaining a high throughput because the substrate assembly may need to be re-polished if it is xe2x80x9cunder-planarized,xe2x80x9d or components on the substrate may be destroyed if it is xe2x80x9cover-polished.xe2x80x9d Thus, it is highly desirable to stop CMP processing at the desired endpoint.
In one conventional method for determining the endpoint of CMP processing, the planarizing period of a particular substrate is determined using an estimated polishing rate based upon the polishing rate of identical substrates that were planarized under the same conditions. The estimated planarizing period for a particular substrate, however, may not be accurate because the polishing rate or other variables may change from one substrate to another. Thus, this method may not produce accurate results.
In another method for determining the endpoint of CMP processing, the substrate is removed from the pad and then a measuring device measures a change in thickness of the substrate. Removing the substrate from the pad, however, interrupts the planarizing process and may damage the substrate. Thus, this method generally reduces the throughput of CMP processing.
U.S. Pat. No. 5,433,651 issued to Lustig et al. (xe2x80x9cLustigxe2x80x9d) discloses an in-situ chemical-mechanical polishing machine for monitoring the polishing process during a planarizing cycle. The polishing machine has a rotatable polishing table including a window embedded in the table. A polishing pad is attached to the table, and the pad has an aperture aligned with the window embedded in the table. The window is positioned at a location over which the workpiece can pass for in-situ viewing of a polishing surface of the workpiece from beneath the polishing table. The planarizing machine also includes a light source and a device for measuring a reflectance signal representative of an in-situ reflectance of the polishing surface of the workpiece. Lustig discloses terminating a planarizing cycle at the interface between two layers based on the different reflectances of the materials. In many CMP applications, however, the desired endpoint is not at an interface between layers of materials. Thus, the system disclosed in Lustig may not provide accurate results in certain CMP applications.
Another optical endpointing system is a component of the Mirra(copyright) planarizing machine manufactured by Applied Materials Corporation of California. The Mirra(copyright) machine has a rotary platen with an optical emitter/sensor and a planarizing pad with a window over the optical emitter/sensor. The Mirra(copyright) machine has a light source that emits a single wavelength band of light.
U.S. Pat. No. 5,865,665 issued to Yueh (xe2x80x9cYuehxe2x80x9d) discloses yet another optical endpointing system that determines the endpoint in a CMP process by predicting the removal rate using a Kalman filtering algorithm based on input from a plurality of Line Variable Displacement Transducers (xe2x80x9cLVDTxe2x80x9d) attached to the carrier head. The process in Yueh uses measurements of the downforce to update and refine the prediction of the removal rate calculated by the Kalman filter. This downforce, however, varies across the substrate because the pressure exerted against the substrate is a combination of the force applied by the carrier head and the topography of both the pad surface and the substrate. Moreover, many CMP applications intentionally vary the downforce during the planarizing cycle across the entire substrate, or only in discrete areas of the substrate. The method disclosed in Yueh, therefore, may be difficult to apply in some CMP application because it uses the downforce as an output factor for operating the Kalman filter.
The present invention is directed toward planarizing machines, control systems for planarizing machines, and method for endpointing or otherwise controlling mechanical and/or chemical-mechanical planarization of microelectronic substrates. In one aspect of the invention, a system for controlling a mechanical or chemical-mechanical planarizing machine comprises a light system, a sensor, and a computer. The light system can have a light source comprising at least a first emitter that generates a first light pulse having a first color and a second emitter that generates a second light pulse having a second color different than the first color. The light source is configured to direct the first and second light pulses toward a front surface of a microelectronic substrate in a manner that creates a first return light pulse corresponding to a reflectance of the first light pulse and a second return light pulse corresponding to a reflectance of the second light pulse. The sensor is configured to receive the first return light pulse and the second return light pulse, and the sensor can generate a first measured intensity of the first return light pulse and a second measured intensity of the second return light pulse. The computer is coupled to the sensor, and the computer may also be coupled to the light source.
The computer has a database and a computer readable medium. The database can contain a plurality of sets of reference reflectances in which each set has a first reference component defined by a reflectance intensity of the first light pulse and a second reference component defined by a reflectance intensity of the second light pulse from a selected surface level in a layer of material on the microelectronic substrate. The computer readable medium can contain a computer readable program that causes the computer to control a parameter of the planarizing machine when the first and second measured intensities correspond to the first and second reference components of a selected reference reflectance set.
The control system described above can have several different embodiments. In one particular embodiment, the light source can further include a third emitter that generates a third source light pulse. For example, the light source can have three emitters such that: (a) the first emitter comprises a red LED that generates a red first light pulse having a wavelength of approximately 600 nm to 780 nm and a red first return light pulse; (b) the second emitter comprises a green LED that generates a green second light pulse having a wavelength of approximately 490 nm to 577 nm and a green second return light pulse; and (c) the third emitter comprises a blue LED that generates a blue third light pulse having a wavelength of approximately 450 nm to 490 nm and a blue third return light pulse. The database can accordingly include an endpoint reference reflectance set having a first reference component corresponding to a first endpoint intensity of the red first return light pulse from an endpoint surface, a second endpoint component corresponding to a second endpoint intensity of the green second return light pulse from the endpoint surface, and a third reference component corresponding to a third endpoint intensity of the blue third return light pulse from the endpoint surface. Additionally, the computer readable program can cause the computer to terminate a planarizing cycle when the first, second and third measured intensities correspond to the first, second and third endpoint intensities, respectively.
Additional aspects of the invention are directed toward methods of planarizing a microelectronic device substrate. One such method in accordance with an embodiment of the invention comprises: contacting a face of the substrate with a planarizing surface of a planarizing pad; moving the substrate and/or the planarizing pad to rub the planarizing surface against the face of the substrate; impinging a first light pulse against the face of the substrate at a first time interval, the first light pulse having a first color; directing a second light pulse against the face of the substrate at a second time interval, the second light pulse having a second color; sensing a first intensity of a first return light pulse corresponding to the first light pulse reflecting from the substrate and a second intensity of a second return light pulse corresponding to the second light pulse reflecting from the substrate; and controlling a parameter of the planarizing cycle of the substrate according to the first and second intensities of the first and second return light pulses.
Another aspect of the invention is a microelectronic substrate assembly for use in controlling mechanical and/or chemical-mechanical planarization processes. One such microelectronic substrate assembly in accordance with an embodiment of the invention comprises a substrate, a first layer over the substrate, a second layer over the first layer, and a sacrificial marking layer or endpoint layer. The first layer is composed of a first material having first color, and the first layer is disposed over at least a portion of the substrate. The first layer also has a first surface defining a desired marking elevation for a planarizing cycle. The second layer is composed of a second material disposed over the first layer, and the second layer has a second color different than the first color. The sacrificial layer is composed of a third material having a third color optically distinct from the first and second colors of the first and second materials. The sacrificial layer, for example, can comprise an opaque resist material. The sacrificial layer can also have a distinct color, such as red, black or white, that has a high optical contrast with the first and second colors of the first and second layers.